In the classical railroad air brake system, as developed from the Westinghouse air brake, the brake air line which passes from the locomotive and then from car to car down the length of the train, provides two basic functions.
First, it is used to charge compressed air tanks in the railroad cars. The air stored in these tanks provides the energy needed to apply the brake shoes when a brake application is required. When the train is running normally, and no brake application is needed, a high pressure in the range from 70 to 110 pounds exists in the brake air line. The tanks in the cars are charged to the same pressure as the air in the brake air line.
Second, when a brake application is required, air is exhausted from the brake air line, causing the pressure in the brake air line to be reduced. In the cars of the train, this reduction of pressure is used as a signal to apply the brakes. In this event, valving in the cars utilizes the compressed air in the tanks to apply pressure to the brake shoes so that the brakes are applied.
After a train has been stopped by an application of the air brakes, the air pressure in the tanks on the cars of the train is depleted. In order for the train to operate safely, the engineer must wait until the tanks are recharged before he puts the train in motion.
In order for the engineer to know when the tanks are charged, a flow meter is used to indicate the flow rate of air from a compressor and reservoir in the locomotive to the brake pipe in the locomotive. When this flow stops, the engineer knows that the tanks in the cars are fully charged, and that it is safe to proceed.
In the prior art system, an orifice is built into the line which supplies air to the main valve which supplies air to the brakepipe in the locomotive. Two pressure transducers are used to measure pressures which can be used to calculate the air flow. One tap is generally upstream of the orifice. Another tap may be in the restricted portion of the orifice.
The difference between these signals is calculated in a computer, which also takes the square root of the difference, and multiplies by a constant, to obtain the flow rate.
This system has the disadvantage that if one or both of the transducers drift, providing a reading not in accordance with its calibration, then a false value will be computed for the airflow. This can be a very dangerous condition, because if a finite airflow rate is read as being zero, the engineer may believe that it is safe to put the train in motion when, in fact, it is not.
The system is quite sensitive to errors caused by drift of the transducers because the step of subtracting one transducer signal from the other causes percentage errors greater than the errors in either transducer separately.
In order to prevent such errors, in the prior art system, regularly-scheduled maintenance is necessary for the transducers. A known pressure is applied to the line which the transducers monitor, with zero airflow in the line. The transducers are then adjusted to give exact readings of the pressure by adjusting potentiometers on them.
This procedure has the disadvantages that it is labor intensive, and that errors may accumulate during the entire period between maintenance checks.
To provide additional information on railroad airbrake systems, as background for the present patent, the teachings of the following United States patents are incorporated herein by reference thereto.
U.S. Pat. No. 4,904,027 by Skantar and Sanders: DIGITAL AIR BRAKE CONTROL SYSTEM.
U.S. Pat. No. 5,192,118 by Balukin, Newingham and Jerina: ELECTRO-PNEUMATIC LOCOMOTIVE BRAKE CONTROL SYSTEM.
U.S. Pat. No. 5,222,788 by Dimsa and Jenets: MICROPROCESSOR BASED ELECTRO-PNEUMATIC LOCOMOTIVE BRAKE CONTROL SYSTEM HAVING BRAKE ASSURANCE CIRCUIT.
Each of these patents is assigned to the assignee of the present invention.